Causes of ozone layer depletion
For nearly a billion years, ozone molecules in the atmosphere have protected life on Earth from the effects of ultraviolet rays.
The ozone layer resides in the stratosphere and surrounds the entire Earth. UV-B radiation (280- to 315- nanometer (nm) wavelength) from the Sun is partially absorbed in this layer. As a result, the amount of UV-B reaching Earth’s surface is greatly reduced. UV-A (315- to 400-nm wavelength) and other solar radiation are not strongly absorbed by the ozone layer. Human exposure to UV-B increases the risk of skin cancer, cataracts, and a suppressed immune system. UV-B exposure can also damage terrestrial plant life, single cell organisms, and aquatic ecosystems. Credit: www.theozonehole.com.
Ozone layer depletion background
Chlorofluorocarbons (CFCs) and other halogenated ozone depleting substances (ODS) are mainly responsible for man-made chemical ozone depletion.
CFCs were invented by Thomas Midgley, Jr. in the 1920s.
They were used in air conditioning and cooling units, as aerosol spray propellants prior to the 1970s, and in the cleaning processes of delicate electronic equipment.
They also occur as by-products of some chemical processes.
No significant natural sources have ever been identified for these compounds—their presence in the atmosphere is due almost entirely to human manufacture.
In 1974 Frank Sherwood Rowland, Chemistry Professor at the University of California at Irvine, and his postdoctoral associate Mario J. Molina suggested that long-lived organic halogen compounds, such as CFCs, would reach the stratosphere where they would be dissociated by ultraviolet light, releasing chlorine atoms.
In 1976 the United States National Academy of Sciences released a report concluding that the ozone depletion hypothesis was strongly supported by the scientific evidence.
Scientists calculated that if CFC production continued to increase at the going rate of 10 percent per year until 1990 and then remain steady, CFCs would cause a global ozone loss of 5–7 percent by 1995, and a 30–50 percent loss by 2050.
In response the United States, Canada and Norway banned the use of CFCs in aerosol spray cans in 1978.
Causes of ozone layer depletion
Ozone depletion describes two distinct but related phenomena observed since the late 1970s.
1) a steady decline in the total amount of ozone in Earth's stratosphere, and
2) a much larger seasonal springtime decrease in stratospheric ozone around Earth's polar regions. The role of sunlight in ozone depletion is the reason why the Antarctic ozone depletion is greatest during spring. During winter there is no light over the pole to drive chemical reactions.
During the spring, however, the sun comes out, providing energy to drive photochemical reactions and melt the polar stratospheric clouds, releasing considerable chlorine-based compounds, which drives ozone damage.
Ozone can be destroyed by a number of free radical catalysts, the most important of which are the hydroxyl radical (OH), nitric oxide radical (NO), chlorine radical (Cl) and bromine radical (Br) - all of these forms have an unpaired electron and are thus extremely reactive.
Once in the stratosphere, ultraviolet light acts to liberate these free radical catalysts from parent compounds. Chlorine for example acts to remove an oxygen atom from ozone creating 'O' and 'O2' - the 'O' then combines with chlorine to produce chlorine monoxide (ClO) and ClO then acts to convert to Cl and 2 sets of O2. Cl is then free to repeat the cycle.
It is calculated that a CFC molecule takes an average of about five to seven years to go from the ground level up to the upper atmosphere, and it can stay there for between 20 -100 years, destroying up to one hundred thousand ozone molecules during that time.
Ozone layer depletion: a happy ending to this story?
The ozone story offers hope for mankind in showing that concerted unilateral action can make a difference.
A gradual trend toward ozone layer
"healing" was widely reported in 2016 based on research by UK and US scientists.
The ozone layer is expected to begin to recover in coming decades due to declining ozone-depleting substance concentrations.
The Antarctic ozone hole is expected to continue for decades but ozone concentrations in the lower stratosphere over Antarctica are expected to increase by 5–10 percent by 2020 and to eventually return to pre-1980 levels by about 2060–2075.
The adoption and strengthening of the Montreal Protocol (which was drawn up 1987 but came into effect in 1989) has to take major credit for this reversal of fortunes.
As a result of the protocol harmful substances are being gradually removed from the atmosphere; since peaking in 1994, the Effective Equivalent Chlorine (EECl) level in the atmosphere had dropped by about 10 percent by 2008.
The phase-out of CFCs means that nitrous oxide (N2O), which is not covered by the Montreal Protocol, has become the most highly emitted ozone-depleting substance and is expected to remain so throughout the 21st century.
A 2005 IPCC review of ozone observations and model calculations concluded that the global amount of ozone has now approximately stabilised.